In a motor driven device such as a machine tool, it is important to know precise locations so that accurate work can be accomplished in a rapid manner. This is particularly true in large, automated production lines wherein capital investment reaches many millions of dollars. Work must be done within close tolerances in a minimum amount of time.
Conventional equipment, using drive motors or clutches to an output shaft, suffers the disability of not knowing precisely where the motor or clutch stops and consequently not knowing the position of the output. Complicated, expensive control schemes are utilized in an attempt to accomplish this. If the position of the tool being driven cannot be accurately established, the work is out of tolerance and must be rejected. In automated, high production lines, seconds of time equate to many thousands of dollars so it is necessary to accomplish accurate work as quickly as possible.
The objective in automated machining is to rapidly move a tool, such as a drill for example, to a position as close as possible to the work piece. The movement of the tool must then be slowed to feed rate so as to engage the work piece in the feed mode. The changeover from rapid traverse to feed must occur at a precise, repeatable location or the tool will "slide over", slamming into the work piece with tool breakage and work piece destruction. If the shift point is backed off to insure against breakage, excessive "air cutting" time is introduced as the tool moves through the air at the feed rate.
The arrangement for linearly advancing and retracting a rotating shaft at two different speeds such as "rapid traverse" and "feed speed" in machine tools can be accomplished in various ways.
One approach uses two different electric motors, one for each mode. The two motor approach has good high speed capability and good high thrust against a stop. However, this approach has a very poor fast duty cycle capability; it has a high parasitic "air cutting" time; it cannot eliminate tool-breaking slide over; it is not versatile in that it cannot set speeds and feeds via a program; it is bulky and occupies excessive floor space; it cannot provide dual platen position verification for safety; and it has the disadvantage of mechanical braking and mechanical limit switches.
A second approach uses a single direct servo drive. This approach has low parasitic time; it eliminates slide over; it is versatile; and it contains no mechanical limit switches or mechanical cycling brake. It has good rapid traverse capability but its maximum thrust capability is poor and it does not have high thrust against a stop; it is bulky; it cannot perform dual platen position verification for safety; and it has a high electrical failure rate due to many electronic components.
A third approach uses a single servo motor with a fixed ratio shiftable transmission. It enables the use of a small motor such as two horsepower; its fast continuous duty cycle capability is good; it has low parasitic time; it eliminates mechanical limit switches and mechanical cycling brakes.
However, maximum rapid traverse capability and high feed thrust must be compromised one for the other according to the single gear ratios selected; dual platen position verification cannot be accomplished; and the electrical failure rate is moderate.
Best results are obtained with a single electric motor capable of moving in rotational increments in combination with a shiftable transmission. This can be an encoded a.c. or d.c motor or a stepper motor. Optimum results are obtained using the stepper motor with feed back of position. Of the systems discussed above, this arrangement has the fastest continuous duty cycle capability. It has low parasitic time; it eliminate slide over; it has good maximum thrust capability and continuous high thrust against a stop; it is versatile; it is compact; it eliminates mechanical limit switches and mechanical cycling brakes; and it has a low failure rate since it uses fewer electronic components.
In addition, the shiftable stepper motor system fails "stopped" so it is virtually "runaway proof". It provides a power-off holding brake with its permanent magnets. It requires a digital pulse train to cause rotation. Even if it takes one step more than desired, this is usually tatamount to a degree or two of rotation and very small linear movement of the machine lead screw shaft. The shiftable stepper motor system has the highest degree of possible computer diagnosis and has the lowest cost for communication interface.